- Kolmogorov space
In

topology and related branches ofmathematics , the**T**or_{0}spaces**Kolmogorov spaces**, named afterAndrey Kolmogorov , form a broad class ofwell-behaved topological space s.The T_{0}condition is one of theseparation axiom s.**Definition**A

**T**is a topological space in which every pair of distinct points is_{0}spacetopologically distinguishable . That is, for any two distinct points "x" and "y" there is anopen set which contains precisely one of the points.Note that topologically distinguishable points are automatically distinct. On the other hand, if the

singleton set s {"x"} and {"y"} are separated, then the points "x" and "y" must be topologically distinguishable. That is,:"separated" ⇒ "topologically distinguishable" ⇒ "distinct"The property of being topologically distinguishable is, in general, stronger than being distinct but weaker than being separated. In a T_{0}space, the second arrow above reverses; points are distinctif and only if they are distinguishable. This is how the T_{0}axiom fits in with the rest of theseparation axiom s.**Examples and counterexamples**Nearly all topological spaces normally studied in mathematics are T

_{0}. In particular, all Hausdorff (T_{2}) spaces and T_{1}spaces are T_{0}.**paces which are not T**_{0}*A set with more than one element, with the

trivial topology . No points are distinguishable.

*The set**R**^{2}where the open sets are the Cartesian product of an open set in**R**and**R**itself, i.e., theproduct topology of**R**with the usual topology and**R**with the trivial topology; points ("a","b") and ("a","c") are not distinguishable.

*The space of allmeasurable function s "f" from thereal line **R**to thecomplex plane **C**such that theLebesgue integral of |"f"("x")|^{2}over the entire real line is finite. Two functions which are equalalmost everywhere are indistinguishable. See also below.**paces which are T**_{0}but not T_{1}*The

Zariski topology on Spec("R"), theprime spectrum of acommutative ring "R" is always T_{0}but generally not T_{1}. The non-closed points correspond toprime ideal s which are not maximal. They are important to the understanding of schemes.

*Theparticular point topology on any set with at least two elements is T_{0}but not T_{1}since the particular point is not closed (its closure is the whole space). An important special case is theSierpiński space which is the particular point topology on the set {0,1}.

*Theexcluded point topology on any set with at least two elements is T_{0}but not T_{1}. The only closed point is the excluded point.

*TheAlexandrov topology on apartially ordered set is T_{0}but will not be T_{1}unless the order is discrete (agrees with equality). Every finite T_{0}space is of this type. This also includes the particular point and excluded point topologies as special cases.

*Theright order topology on atotally ordered set is a related example.

*Theoverlapping interval topology is similar to the particular point topology since every open set includes 0.

*Quite generally, a topological space "X" will be T_{0}if and only if thespecialization preorder on "X" is apartial order . However, "X" will be T_{1}if and only if the order is discrete (i.e. agrees with equality). So a space will be T_{0}but not T_{1}if and only if the specialization preorder on "X" is a non-discrete partial order.**Operating with T**_{0}spacesExamples of topological space typically studied are T

_{0}.Indeed, when mathematicians in many fields, notably analysis, naturally run across non-T_{0}spaces, they usually replace them with T_{0}spaces, in a manner to be described below. To motivate the ideas involved, consider a well-known example. The space L^{2}(**R**) is meant to be the space of allmeasurable function s "f" from thereal line **R**to thecomplex plane **C**such that theLebesgue integral of |"f"("x")|^{2}over the entire real line is finite.This space should become anormed vector space by defining the norm ||"f"|| to be thesquare root of that integral. The problem is that this is not really a norm, only aseminorm , because there are functions other than thezero function whose (semi)norms are zero.The standard solution is to define L^{2}(**R**) to be a set ofequivalence class es of functions instead of a set of functions directly.This constructs aquotient space of the original seminormed vector space, and this quotient is a normed vector space. It inherits several convenient properties from the seminormed space; see below.In general, when dealing with a fixed topology

**T**on a set "X", it is helpful if that topology is T_{0}. On the other hand, when "X" is fixed but**T**is allowed to vary within certain boundaries, to force**T**to be T_{0}may be inconvenient, since non-T_{0}topologies are often important special cases. Thus, it can be important to understand both T_{0}and non-T_{0}versions of the various conditions that can be placed on a topological space.**The Kolmogorov quotient**Topological indistinguishability of points is an

equivalence relation . No matter what topological space "X" might be to begin with, thequotient space under this equivalence relation is always T_{0}. This quotient space is called the**Kolmogorov quotient**of "X", which we will denote KQ("X"). Of course, if "X" was T_{0}to begin with, then KQ("X") and "X" are naturallyhomeomorphic .Categorically, Kolmogorov spaces are areflective subcategory of topological spaces, and the Kolmogorov quotient is the reflector.Topological spaces "X" and "Y" are

**Kolmogorov equivalent**when their Kolmogorov quotients are homeomorphic. Many properties of topological spaces are preserved by this equivalence; that is, if "X" and "Y" are Kolmogorov equivalent, then "X" has such a property if and only if "Y" does.On the other hand, most of the "other" properties of topological spaces "imply" T_{0}-ness; that is, if "X" has such a property, then "X" must be T_{0}.Only a few properties, such as being anindiscrete space , are exceptions to this rule of thumb.Even better, many structures defined on topological spaces can be transferred between "X" and KQ("X").The result is that, if you have a non-T_{0}topological space with a certain structure or property, then you can usually form a T_{0}space with the same structures and properties by taking the Kolmogorov quotient.The example of L

^{2}(**R**) displays these features.From the point of view of topology, the seminormed vector space that we started with has a lot of extra structure; for example, it is avector space , and it has a seminorm, and these define apseudometric and auniform structure that are compatible with the topology.Also, there are several properties of these structures; for example, the seminorm satisfies theparallelogram identity and the uniform structure is complete. The space is not T_{0}since any two functions in L^{2}(**R**) which are equalalmost everywhere are indistinguishable with this topology.When we form the Kolmogorov quotient, the actual L^{2}(**R**), these structures and properties are preserved.Thus, L^{2}(**R**) is also a complete seminormed vector space satisfying the parallelogram identity.But we actually get a bit more, since the space is now T_{0}.A seminorm is a norm if and only if the underlying topology is T_{0}, so L^{2}(**R**) is actually a complete normed vector space satisfying the parallelogram identity — otherwise known as aHilbert space .And it is a Hilbert space that mathematicians (andphysicists , inquantum mechanics ) generally want to study. Note that the notation L^{2}(**R**) usually denotes the Kolmogorov quotient, the set ofequivalence class es of square integrable functions which differ on sets of measure zero, rather than simply the vector space of square integrable functions which the notation suggests.**Removing T**_{0}Although norms were historically defined first, people came up with the definition of seminorm as well, which is a sort of non-T

_{0}version of a norm. In general, it is possible to define non-T_{0}versions of both properties and structures of topological spaces. First, consider a property of topological spaces, such as being Hausdorff. One can then define another property of topological spaces by defining the space "X" to satisfy the property if and only if the Kolmogorov quotient KQ("X") is Hausdorff. This is a sensible, albeit less famous, property; in this case, such a space "X" is called "preregular". (There even turns out to be a more direct definition of preregularity). Now consider a structure that can be placed on topological spaces, such as a metric. We can define a new structure on topological spaces by letting an example of the structure on "X" be simply a metric on KQ("X"). This is a sensible structure on "X"; it is apseudometric . (Again, there is a more direct definition of pseudometric.)In this way, there is a natural way to remove T

_{0}-ness from the requirements for a property or structure. It is generally easier to study spaces that are T_{0}, but it may also be easier to allow structures that aren't T_{0}to get a fuller picture. The T_{0}requirement can be added or removed arbitrarily using the concept of Kolmogorov quotient.**External links*** [

*http://www.mathematik.tu-darmstadt.de:8080/Math-Net/Lehrveranstaltungen/Lehrmaterial/SS2003/Topology/separation.pdf History of weak separation axioms*] (PDF file)

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